This invention relates to a process for the manufacture of N-chlorophthalimide. The prior art processes for preparing N-haloimide compounds, and, specifically, N-chloroimide compounds, customarily involve the use of an aqueous system. In general, prior art processess for preparing N-chloroimides can be classified as follows:
(1) Chlorination of the corresponding imide using an inorganic hypochlorite in a mixture of acetic acid and water;
(2) Chlorination by passing chlorine into an aqueous solution comprising equivalent amounts of the corresponding imide and a strong base, e.g., sodium hydroxide or potassium hydroxide;
(3) Chlorination of the corresponding imide using t-butyl hypochlorite in a mixture of t-butyl alcohol and water.
Of the above general methods, only method (2) prescribes the use of chlorine itself in the production of the N-chloroimide. However, due to the fact of the aqueous system, this method has been found to have serious drawbacks. First, chlorine is only very slightly soluble in water. Secondly, and more importantly, it is known that an imide, when present in an alkaline aqueous medium such as would result from potassium or sodium hydroxide and water, undergoes rapid hydrolysis. When, for example, phthalimide is subjected to alkaline aqueous conditions, the following decomposition sequence occurs: ##STR1##
Even more importantly, it has been established [Arthur R. Hurwitz, "Degradation of N-Chlorosuccinimide in Aqueous Solution", Diss. Abst., B, 28 (3), 971 (1967)] that an N-chloroimide product, when present in an aqueous alkaline medium, such as would be the case under the conditions of chlorinations provided by method (2) above, degrades with possible formation of the highly explosive and toxic gas, nitrogen trichloride. The following sequences are postulated for the decomposition of N-chlorosuccinimide: ##STR2##
The process of this invention uses molecular chlorine as chlorinating agent and provides the preparation of N-chlorophthalimide both in high yield and high purity and without substantial product degradation due to alkaline hydrolysis. The reaction is carried out in a substantially non-aqueous medium, and the aforementioned deficiencies of an aqueous alkaline medium thus are avoided.
Therefore, this invention is directed to a process for preparing N-chlorophthalimide which comprises contacting an alkali metal salt of phthalimide with chlorine under substantially non-aqueous conditions in the presence of a halogenated aliphatic hydrocarbon and at a temperature of from about -10.degree. C. to about +40.degree. C.
Although the customary prior art methods for preparing N-haloimides are as described above, the literature in an isolated instance does describe a non-aqueous method for preparing N-bromophthalimide. This method, described in J. Bredt and H. Hof, Berichte, 33, 21 (1900), involves treating potassium phthalimide with molecular bromine in an inert non-aqueous solvent. The authors describe the method as unsuitable, and they expressly reject this method in favor of the typical prior art method using bromine in an aqueous alkaline medium.